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Surface patterns of the low-frequency current in a 20 × 30 km region in front of the Venice Lagoon were analysed from a 13-month-long HF radar data set. Surface circulation was related to prevalent wind regimes in the area and to the tidal flow through the lagoon inlet. Three different categories of wind-forcing were defined: bora (NE wind), sirocco (SE wind), and finally the category containing all other wind directions and calms (winds lower than 3 m/sec). Mean flow and vorticity spatial distributions were discussed for different wind conditions. The coastal area about 5 km wide is characterized by a flow field with maximum vorticity. Outside the coastal boundary layer the interior flow is part of the Adriatic basin-wide cyclonic circulation. Two counter-rotating vortices of the dimension of about 4–5 km were evidenced in the average flow field in front of the lagoon inlet (Malamocco inlet) in all situations except for the bora. The vortex-pair is probably associated with the tidal flow through the inlet. The bora wind induces a strong southward coastal jet detached from the coast by about 5 km homogenizing the flow and eliminating the dipole. The average coastal flow pattern in calm wind conditions was also analysed as a function of the strong inflow/outflow (currents in the inlet > 0.7 m s-1) from the lagoon inlet. In both cases the vorticity pattern is similar, with the negative vorticity to the left of the inlet and positive to the right looking seaward.

The feasibility of monitoring the suspended particle concentration (SPC) and fluxes (PF) with Acoustic Doppler Current Profilers (ADCP) was tested in the Venice Lagoon inlets. The backscatter recorded by boat- and bottom-mounted instruments was converted into SPC estimates with a calibration procedure based on multiple water sample collection and ADCP transects. Time series of PF across the inlets were derived from discharge and SPC data allowing for the calculation of a preliminary budget of the exchanges between the lagoon and the open sea.

Lo studio della circolazione costiera e dello scambio di flussi d’acqua tra la laguna di Venezia ed il mare aperto attraverso le sue bocche di porto è stato uno dei temi principali di un progetto di ricerca, finanziato dal CORILA (Consorzio Ricerche Lagunari) negli anni 2001-2004 Un aspetto preso in considerazione in questa ricerca è il ruolo fondamentale che le correnti marine superficiali giocano nell’ambiente costiero per la loro influenza nell’ecosistema, e come il campo di corrente adiacente alle bocche, nella fascia costiera fuori della laguna, interviene nell’efficienza dei flussi attraverso le bocche nel rinnovare le acque della laguna. Lo studio della circolazione superficiale e del suo impatto sulle caratteristiche ambientali richiede l’acquisizione di lunghe serie di dati di corrente di alta qualità, che spesso risultano carenti a causa delle limitazioni nelle tecniche di misura disponibili. Una nuova tecnica di misura basata su una rete di sistemi Radar HF per produrre le mappe della corrente marina superficiale al largo della bocca di porto di Malamocco, è stata utilizzata, permettendo l’analisi della completa serie temporale dal 2001 al 2003. Questa tecnologia, già ampiamente utilizzata negli Stati Uniti e sperimentata per la prima volta in Italia dal gruppo di ricerca del CNR – ISMAR di Venezia, in collaborazione con l’OGS di Trieste ha permesso di superare le difficoltà fino ad ora incontrate nello studio delle correnti in ampie aree di mare. La circolazione costiera é stata studiata con l’utilizzo di radar costieri ad alta frequenza (25-30 MHz). Il radar utilizza due sistemi di antenne ricetrasmittenti posizionate sulle isole di Lido e di Pellestrina, più una terza antenna Rx/Tx installata sulla piattaforma “Acqua Alta”, 8 miglia al largo delle bocche di porto. Con questa configurazione é stata coperta dalle misure una zona larga 20km davanti alla bocca di Malamocco, con una risoluzione spaziale di 1km, ottenendo le serie temporali desiderate.

A network of high frequency (HF) radar systems was used to produce maps of surface velocity offshore of the Malamocco inlet during the 12-month period from November 2001 through October 2002. Inspection of the sub-tidal residual flow fields revealed frequent occurrences of small-scale (~10 km) eddy structures both north and south of the inlet. In addition, several monthly averaged velocity maps indicate the presence of a persistent meander in the mean current patterns. A more objective technique, based on vorticity, was developed to search the data set for the presence of eddies near the inlet and to separate those features from the larger-scale meander pattern. The vorticity at scales of 5-10 km was computed each hour using the low-passfiltered data set and year-long vorticity statistics were investigated at selected locations surrounding the inlet. The vorticity histograms and temporal variability changed significantly as a function of location relative to the Malamocco inlet. Both north and south of the inlet, the mean vorticity was weakly negative reflecting the larger-scale meander pattern whose crest is aligned, approximately, with the inlet. However, the range of vorticity north of the inlet was much greater than it was south of the inlet. A minor peak (or shoulder) in the year-long histogram of vorticity north of the inlet highlighted frequent occurrences of strong (~ 5 × 10-5 s-1) positive vorticity events in that area. A similar statistical behaviour was not seen south of the inlet. This observation was used to define a conditional sampling criteria that lead to a map of the typical flow field associated with strong eddy events. The vorticity time series were also used to investigate the relationship of eddy events to external forcing parameters. Strong winds, for instance, acted to destroy vorticity in the surface current field since they lead to a strong but horizontally uniform response in the surface currents. The roles of sea level variability and offshore current velocity were also investigated.

The CODAR (Coastal Ocean Dynamics Application Radar) system, was installed for the first time in the Adriatic Sea off the Ancona coast (Italy). The system operated continuously, from September 1997 through February 2000. Some results of the data analysis on a monthly time-scale for one whole year (September 1997-August 1998) are shown. They corroborate a presence of a 10-15 km wide coastal flow, known as WAC (Western Adriatic Current), prevalently flowing towards the south-east. Its variability on a seasonal time scale is clearly evident. A possible influence on the WAC of the freshwater discharge from the Po River (acting through a horizontal pressure gradient) and its spreading in the Northern Adriatic basin is documented. A freshwater discharge confined to the western coast is associated to a stronger horizontal pressure gradient which then reinforces the WAC. On the contrary, fresh water dissipated in the surface layer towards the eastern coast and trapped in the northernmost sub-basin of the Adriatic, as may occur during summer, is associated to the much weaker WAC. Some hints of a synoptic time scale variability in the current field are reported, putting into evidence a reversal of the prevalent south-eastward current observed in some meteorological situations characterized by a SE (sirocco) wind.